Cavity receiver in solar tower concentrator usually experiences highly intense radiation. Due to asymmetric concentration of solar rays, non-uniform heat flux distribution occurs on the different parts of the cavity receiver. This non-uniform distribution leads to uneven thermal expansion and stresses in receiver, which affects the reliable operation and reduces life time of receiver parts. Therefore, it is necessary to reduce the non-uniformity of solar flux on the surface of the absorber tubes and different parts of the solar reactor. The aim of this study was to focuses on the distributions of concatenated solar flux over graphite tubes of a 50kW solar reactor, which was previously designed for methane thermal dissociation at the focus of a solar furnace. In this study, the absorbed solar power on the different parts of the reactor is determined by Monte Carlo ray tracing method. Moreover, the effect of aperture size and the absorptivity of receiver parts on the net magnitude and distribution of absorbed power in reactor are investigated. The results prove that the 16cm aperture absorbs the maximum power and leads to even better solar flux distributions. Replacing the absorbing walls by the reflective walls will also result in more power absorbed by the tubes and better uniformity of flux distribution around the tubes.